1. Field of the Invention
The invention relates to tensioning devices for exerting a tensile force from a drilling vessel or drilling platform upon a drilling or production riser.
2. Description of Related Art
A marine riser system is employed to provide a conduit from a floating vessel at the water surface to the blowout preventer stack or, production tree, which is connected to the wellhead at the sea floor. A tensioner, or motion compensator, is incorporated into the riser string to compensate for vessel motion induced by wave action and heave. A tensioning system is utilized to maintain a variable tension to the riser string alleviating the potential for compression and in turn buckling or failure.
Historically, conventional riser tensioner systems have consisted of both single and dual cylinder assemblies with a fixed cable sheave at one end of the cylinder and a movable cable sheave attached to the rod end of the cylinder. The assembly is then mounted in a position on the vessel to allow convenient routing of wire rope which is connected to a point at the fixed end and strung over the movable sheaves. In turn, the wire rope is routed via additional sheaves and connected to the slip-joint assembly via a support ring consisting of pad eyes which accept the end termination of the wire rope assembly. A hydro/pneumatic system consisting of high pressure air over hydraulic fluid applied to the cylinder forces the rod and in turn the rod end sheave to stroke out thereby tensioning the wire rope and in turn the riser.
The number of tensioner units employed is based on the tension necessary to maintain support of the riser and a percentage of overpull which is dictated by met-ocean conditions i.e., current and operational parameters including variable mud weight, etc.
Available space for installation and, the structure necessary to support the units including weight and loads imposed, particularly in deep water applications where the tension necessary requires additional tensioners poses difficult problems for system configurations for both new vessel designs and upgrading existing vessel designs.
Recent deepwater development commitments have created a need for new generation drilling vessels and production facilities requiring a plethora of new technologies and systems to operate effectively in deep water and alien/harsh environments. These new technologies include riser tensioner development where reduced weight and required space are important factors to the drilling contractor.
The tensioner assemblies of the present invention offer operational advantages over conventional methodologies by providing options in riser management and current well construction techniques. Applications of the basic module design are not limited to drilling risers and floating drilling vessels. The system further provides cost and operational effective solutions in well servicing/workover, intervention and production riser applications. These applications include all floating production facilities including, tension leg platform, floating production facility, and production spar variants. The system when installed provides an effective solution to tensioning requirements and operating parameters. An integral control and data acquisition system provides operating parameters to a central processor system which provides supervisory control.
Generally, tensioner assemblies are of two types, the piston type and the ram type. With the piston type cylinder, the rod is stroked out by pressured hydraulic fluid which is stored in an external accumulator charged with high pressure air. The hydraulic fluid flows into the cylinder from an external accumulator and the pressurized hydraulic fluid acts on the piston to extend the rod. The piston has a pressure barrier seal between the piston and the inner wall of the cylinder. When the rod is retracted the hydraulic fluid is displaced by the piston and rod flowing back into the external accumulator.
Prior ram-type tensioner assemblies include a ram, which is sealed around its outer diameter to the upper gland of the cylinder. As the pressurized hydraulic fluid flows into the cylinder from the external accumulator the ram extends. When the ram retracts, the hydraulic fluid is displaced back into the external accumulator. Therefore, these prior tensioner assemblies require the hydraulic fluid volume to be displaced by the piston or ram, which then flows back into the external accumulator.
The present invention is directed to ram-type tensioner assemblies in which the hydraulic fluid accumulator is integral with the cylinder and the ram and which includes an air transfer tube disposed within the cylinder cavity and the ram cavity to provide an air over hydraulic fluid arrangement. In this arrangement, the tensioner assemblies of the present invention provide the advantage of reducing the amount of deck space required for each tensioner assembly because external hydraulic fluid accumulators are not necessary. The tensioner assemblies of the present invention also provide that the volume occupied by the wall thickness of the ram displaces the hydraulic fluid. This results in a relatively small rise and fall of the fluid level in the hollow ram, thus eliminating the necessity for an external accumulator. Additionally, the tensioner assemblies of the present invention have reduced weight and require minimal modifications to rig structure as a result of the reduced weight. Moreover, less hydraulic fluid and less high pressure air or gas are required as compared to conventional tensioners.